1. Technical Field
The invention relates to the field of radio frequency (R.F.) circuits and more particularly to an R.F. antenna switch.
2. Discussion of the Prior Art
R.F. switches are commonly used in R.F. applications such as R.F. transceivers known from WO 88/00760. WO 88/00760 concerns R.F. transceivers having an R.F. antenna switch for switching an antenna path and an R.F. local oscillator switch for switching a local oscillator signal path.
The R.F. antenna switch is used to route a common antenna path to either a receiver block or a transmitter block of the R.F. transceiver. The antenna switch essentially consists of two PIN diode switching elements and a one-quarter wavelength transmission line. When the two PIN diodes are switched on, a transmitter port is connected to an antenna port through the first PIN diode and a receiver port is connected to ground through the second PIN diode. When the two PIN diodes are switched off, the transmitter port is disconnected from the antenna port and the receiver port is connected to the antenna port through the transmission line.
The R.F. local oscillator switch according to WO 88/00760 is used to route the local oscillator signal to either a mixer of the receiver block or to an exciter of the transmitter block. The local oscillator switch essentially consists of two PIN diode switching elements and an unequal power splitter in the form of a directional coupler.
The directional coupler has a primary line and a coupled line. Due to the small overall insertion loss of the primary line a receiver signal can be coupled from an input port of the primary line to an output port of the primary line with hardly any losses (low-loss path). Due to a relatively low coupling factor, however, the local oscillator signal transmitted from an input port of the coupled line to the output port of the primary line is strongly attenuated (high-loss path).
The input port of the primary line is connected to the output of a receiver front end circuit and the output port of the primary line is connected to the mixer. The input port of the coupled line is connected to both a local oscillator and an input of a first PIN diode. An output of the first PIN diode is coupled to the exciter. An output port of the coupled line is connected to both an output of a second PIN diode and a terminating element allowing termination of the output port of the coupled line with the characteristic impedance of the coupled line.
When the two PIN diodes are switched on, the output port of the coupled line is shorted and the one-quarter wavelength coupled line has an infinite impedance at the input port of the coupled line. Consequently, the local oscillator signal is routed through the second PIN diode to the exciter. When the two PIN diodes are switched off, the output port of the coupled line is terminated in its characteristic impedance. The local oscillator signal can then be routed through the coupled line and a portion of it is thus transmitted to the output port of the primary line.
The R.F. local oscillator switch according to WO 88/00760 consists of two PIN diodes and a plurality of passive components. However, the plurality of PIN diodes and passive components limits the degree to which the R.F. local oscillator switch can be miniaturized and increases the overall cost of the switch.
The R.F. local oscillator switch, which is used in addition to the above described R.F. antenna switch, is part of a mixer stage allowing to combine two signals. The R.F. local oscillator switch is thus not suited as an R.F. antenna switch for routing either one of two antenna signals to a common port or for routing an antenna signal from the common port to either one of a high-loss port and a low-loss port.
A further R.F. antenna switch is known from WO 97/23929. This R.F. antenna switch is configured as a mechanical component and arranged within a housing of a coaxial accessory connector. A standard antenna is mounted in a top port of the coaxial accessory connector. A radial port in the coaxial accessory connector can be coupled to a further antenna which activates the R.F. switch to disengage from the standard antenna contact and to engage a contact in the radial port. In this way, the R.F. signal is conducted to the further antenna, rather than to the standard antenna. When the R.F. switch is not activated by the further antenna, the R.F. signal is carried through the coaxial accessory connector to the standard antenna.
There is a need for an R.F. antenna switch which has a simple construction and which allows a higher degree of integration. There is also a need for a preferred use of such an R.F. antenna switch and for an R.F. device incorporating such an R.F. antenna switch.
The present invention satisfies these needs by providing an R.F. antenna switch for coupling a common port to either a high-loss port or a low-loss port, the R.F. antenna switch having an unequal power splitter with at least the common port, the high-loss port and the low-loss port. The unequal power splitter further comprises a high-loss path coupled between the common port and the high-loss port and a low-loss path coupled between the common port and the low-loss port. A switching element of the R.F. antenna switch has an input coupled to the low-loss port and an output coupled to a first terminating element. The low-loss port is terminated with the characteristic impedance of the low-loss path when the switching element is switched on.
When the switching element is switched on, the low-loss port has a non-reflecting, i.e., absorbing characteristic. The impedance of the terminating element is, therefore, preferably chosen such that the total impedance of all components coupled to the low-loss port including the impedance of the terminating element equals the characteristic impedance of the low-loss path.
The R.F. antenna switch according to the invention can be realized with only one switching element and few passive components. This not only reduces the cost of the R.F. antenna switch but also allows a higher degree of integration and further miniaturization of the R.F. antenna switch.
Moreover, unequal power splitters comprising e.g. transmission line structures can be realized with ceramic multi-layer technology in an efficient way. Any additional components of the R.F. antenna switch like the switching element or terminating elements can remain discrete components which may be placed on top of the ceramic multi-layer substrate as already practised for state of the art transmitter/receiver-switch modules. Filter structures can advantageously be realized on the same ceramic multi-layer substrate as the R.F. antenna switch. This enables further integration and cost reduction.
The R.F. antenna switch according to the invention can advantageously be used in many technical fields and above all in all kinds of transmitting and receiving applications like positioning system receivers adapted to a standard taken from the group consisting of GPS, GLONASS, BAAS, etc or mobile phones adapted to a standard taken from the group consisting of GSM 900, GSM 1800, GSM 1900, AMPS, PDC, CDMA, WCDMA, DAMPS, etc.
The R.F. antenna switch according to the invention enables to couple either a low-loss port or a high-loss port to a common port. Thus, the R.F. antenna switch allows to couple either a signal applied to the low-loss port or a signal applied to the high-loss port to the common port or to couple a signal applied to the common port either to the low-low port or the high-loss port. Preferably, the R.F. antenna switch is switched by means of changing the impedances at the individual ports of the R.F. antenna switch. Therefore, In order to couple the signal via a specific port, this specific port can be terminated with a characteristic impedance. On the other hand, in order to block a signal from being transmitted via the specific port, the specific port can be terminated with an impedance mismatch. The impedance mismatch is e.g. created by simply switching off an electrical component, like an amplifier stage coupled to the specific port, or by physically disconnecting an electrical component like an antenna coupled to the specific port.
According to a first embodiment, the R.F. antenna switch is used as an antenna switch for coupling the common port to either a first antenna connected to the low-loss port or a second antenna connected to the high-loss port. According to a second embodiment, the R.F. antenna switch is used for coupling either the low-loss port or the high-loss port to an antenna which is connected to the common port. Due to the high signal attenuation of the high-loss path, in both embodiments an active antenna can be coupled to the high-loss port.
The R.F. antenna switch of the invention may have a plurality of high-loss paths and low-loss paths. An R.F. antenna switch having such a plurality of signal paths can advantageously switch between more than two R.F. signals. Thus, novel applications of the R.F. antenna switch of the invention become feasible.
According to a preferred embodiment, the unequal power splitter is a directional coupler with a primary line and a coupled line. The common port and the low-loss port of the directional coupler are coupled to opposite ends of the primary line and the high-loss port of the directional coupler is coupled to one of the coupled lines.
In the directional coupler, microwave power propagating on the coupled line couples uni-directionally to the primary line causing microwave power to appear on it. However, due to low coupling factors only a small portion of the microwave power propagating on the coupled line is transmitted to the primary line and the remaining power continues to propagate on the coupled line. Because of the small amount of microwave power transmitted from the high-loss port of the coupled line to the common port of the primary line or vice versa, this signal path is known as high-loss path. On the other hand, due to comparatively low insertion losses between the common port and the low-loss port which connect the opposing ends of the primary line, this signal path is known as low-loss path.
The primary line and the coupled line can be realized as two closely spaced microstrip transmission lines. Each of the transmission lines preferably has a length corresponding to approximately one-quarter of the signal wavelength. Instead of transmission line structures the directional coupler may also comprise corresponding lumped component equivalents.
The directional coupler can have a isolated port arranged on an opposite end of the coupled line with regard to the high-loss port. This isolated port coupled to one end of the coupled line is preferably coupled to a further terminating element which terminates the isolated port with the characteristic impedance of the coupled line. Therefore, the isolated port has a non-reflecting characteristic for any signals or signal components propagating on the coupled line.
The isolated port coupled to the coupled line and the low-loss port coupled to the primary line can electrically be arranged on opposite sides of the directional coupler with respect to the coupling direction. This arrangement of the low-loss port and the isolated port ensures that in case of uni-directional couplers the isolated port absorbs any signal components input through the low-loss port and transmitted by the directional coupler to the isolated port. With regard to the physical arrangement of the isolated port and the low-loss port, these ports may be arranged either on the same side or on opposite sides of the directional coupler.
According to one embodiment, the R.F. antenna switch further comprises a control circuit for controlling the switching element. The switching element can be e.g. a switching transistor, a varactor diode or a PIN diode.
The control circuit may consist of one or more components and can be coupled to the switching element in many ways. The control circuit is preferably coupled to the common port or the low-loss port of the unequal power splitter.
The control circuit may have a control input terminal for inputting a control signal like a control voltage. By means of the control signal the switching element can be switched between an on position and an off position. Moreover, the control circuit may have a low-pass filter coupled between the control input terminal and the switching element. The low-pass filter ensures that R.F. signals cannot leak through the control circuit.
In many cases it is advantageous to have a supply circuit for supplying a voltage or a current to e.g. an active component which is also coupled to the coupled line. This active component can be an active antenna, an amplifier stage or a signal generator. The supply circuit is coupled to the coupled line of the directional coupler. Preferably, the supply circuit is coupled to the high-loss port or the isolated port of the directional coupler. The supply circuit can further have a supply input terminal for inputting the supply voltage. The supply circuit may also have a low-pass filter coupled between the supply input terminal and either the high-loss port or the isolated port of the directional coupler. The low-pass filter prevents R.F. signals from leaking through the supply circuit.
When the R.F. antenna switch according to the invention is implemented in an R.F. device like a mobile phone for at least one of sending and receiving an R.F. signal, this R.F. device may further comprise one or more external antenna connectors for connecting external antennas to the R.F. antenna switch. The external antenna connectors can e.g. be coupled to at least one of the high-loss port, the low-loss port and the common port of the unequal power splitter. The R.F. device can also comprise one or more antennas. These antennas are preferably either external antennas coupled to at least one of the external antenna connectors of the R.F. antenna switch or internal antennas coupled to at least one of the high-loss port, the low-loss port and the common port of the unequal power splitter. According to a preferred embodiment, the antennas of the R.F. device are designed as mobile phone antennas or positioning system antennas.
The antenna which is coupled directly, or via the external antenna connector, to the high-loss port of the unequal power splitter is preferably an active antenna or an antenna providing high gain. This is advantageous since the high-loss port of the unequal power splitter is coupled to the high-loss path so that the microwave power propagating on the high-loss path from or to the antenna is strongly attenuated.